3D printing (3DP) methods build structures from a digitally designed 3D model. 3DP enables manufacturing structures with configurations with different structural parameters without the need to make a mold for each structure. This feature of 3DP make this method one of the most promising fabrication methods suitable for topology optimization.
Many different types of 3D printing methods (e.g., fused deposition modeling (FDM), selective laser sintering (SLS), stereolithography, solvent cast 3DP (SC3DP) and UV assisted 3DP (UV3DP)) have been developed so far. 3D printing of conductive materials has always been a challenge because the most frequently used conductive materials are metals. Due to their high melting temperature, their utilization as an ink for 3DP methods involving melting and extruding the material from a nozzle (i.e., FDM) is challenging. M. D. Dickey and his co-workers have reported direct-write 3D printing of metallic structures by extrusion of liquid metal from a nozzle. SLS has been used for fabrication of metallic structures by sintering of metal powder using heating originated from a laser beam. Other efforts have been done on printing of conductive polymer based nanocomposite inks using FDM, SC3DP and UV3DP. Conductive structures with electrical conductivity of ˜10 S/m were made by FDM method using a carbon black/polycaprolactone composite ink. Since SC3DP and UV3DP can function at room temperature, they are not subject to the problems caused by the variations in melting point and viscosity due to the addition of fillers. Scaffolds from a graphene-based material with an electrical conductivity of 278 S/m were fabricated using a 3D printing method suitable for printing of aerogels.
Printing of conductive nanocomposites with electrical conductivity of ˜100 S/m was reported for carbon nanotube (CNT)/polylactic acid (PLA) inks. However, increasing the concentration of CNT in such composites to more than 10 wt. % is challenging due to mixing difficulties. The high viscosity of the mixing materials and difficulties related to the dispersion of CNTs at high concentrations in a solvent hinders extrusion and solution mixing, respectively. On the other hand, the fabrication of highly conductive ink from polymer-based composite inks is highly demanding and hardly accessible due to extrusion difficulties of highly doped nanocomposite inks from fine nozzles. Composite inks with high concentrations of conductive fillers have different viscosity behavior which blocks the printing nozzle in 3D printing methods involving melting and extruding an ink, such as fusion deposition modeling (FDM), which is the most popular 3D printing method.